1. Field of the Invention
This invention relates generally to an oscillator circuit, and more particularly to an integrated circuit relaxation oscillator circuit having a relatively stable frequency.
2. Prior Art
Conventional integrated circuit relaxation oscillators generally exhibit relatively large variations in the period of their oscillatory output signal as a result of variations in transistor parameters due to the manufacturing process which produced the transistors and variations in the voltage level of their power supply. Wherever used herein, the terms "process parameters" are intended to include manufacturing process parameters. Those process parameters which provide the largest variations are the enhancement threshold voltage and the mobility and gate capacitance of field effect transistors (FET). Since the threshold voltage of an enhancement FET is that gate-source voltage which biases the FET at a transition between a conductive and a nonconductive state thereof, it can be appreciated that any variation in threshold voltage will have an appreciable influence on the operating characteristics of that FET. The threshold voltage of an FET may vary from -0.9 to -2.1 volts. Accordingly, it can be appreciated that such a variation will have an adverse affect on the period of the oscillatory output signal, unless circuit techniques are employed for cancelling or reducing the effects of variations in threshold voltage.
Since the resistance of an FET does not vary significantly with variations in the voltage level of the power supply connected thereto, the time required to charge or discharge each node of an oscillator circuit varies with variations in the voltage level of the power supply. If the time required to charge or discharge a particular node of the oscillator circuit varies under such conditions, obviously the period of the oscillatory output signal will also vary. Frequency variations of 20 to 1 can be expected in prior known integrated circuit oscillators as a result of process parameter variations and power supply variations. One known integrated circuit oscillator having an output frequency of 400 KHz has a frequency range of from 150 KHz to 900 KHz due to variations in process parameters.
Previous oscillators also exhibited start up problems. That is, relatively small variations in the process parameters of prior known oscillator could result in that oscillator being incapable of starting. Also, race condition problems exist in some of these prior known oscillators.
Previous integrated circuit oscillators were also susceptible to the problem of having more than one stable operating frequency. Usually, this problem is encountered when the voltage level of the oscillator power supply drops below a predetermined amplitude. This problem also occurs in some of the prior known integrated circuit oscillators when the power supply voltage is turned on and applied to the oscillator at a relatively slow rate.
It can be appreciated, therefore, that a need exists for an integrated circuit oscillator which exhibits a relatively small variation in frequency with variations in process parameters and/or variations in power supply levels. A need also exists for such an oscillator which has no start up problems, only one stable operating frequency, and is not subject to race conditions.